Metallic components are used in a variety of industrial products. In particular, magnesium and aluminum components, which are lightweight and exhibit high specific strength, excellent machinability, and high recyclability, are widely used as parts for automobiles, electric appliances such as mobile phones and personal computers, and aircrafts. In general, protective layers are formed on surfaces of the metallic components for the purpose of corrosion control and improvements in coating adhesion. In recent years, communication trouble and adverse effects on humans caused by electromagnetic wave have been problematic. For achieving high electromagnetic wave shielding, protective coatings with low electrical resistance are desired in many cases.
Various methods other than chromate conversion coating have been reported for applying protective coating to surfaces of metallic components such as magnesium components and aluminum components; however, methods using coating solutions containing environmentally hazardous chromium have been avoided. Accordingly, chromium-free treatments have been proposed, examples of which include technologies as disclosed in the following patent literatures.
Japanese Unexamined Patent Application Publication No. 2000-96255 discloses a method for treating a surface of a magnesium-containing metal coated with a low-resistance film, comprising treating the magnesium-containing metal with a high-alkali solution after etching with an acidic and/or weakly alkaline solution; and treating the metal with a chemical conversion solution containing calcium ions, manganese ions, phosphate ions, and an oxidizing agent.
Japanese Unexamined Patent Application Publication No. 2002-12980 discloses a method for producing a magnesium or magnesium alloy component, comprising treating the component with (A) a surface treatment agent containing a phosphate salt and then treating the component with (B) an antirust pretreatment agent.
WO 2003/069024 discloses a magnesium conversion coating composition comprising (a) a source of vanadate anions; (b) a phosphorus-containing material; and (c) a source of nitrate anions, wherein the vanadate anions, phosphorus-containing material, and nitrate anions are dissolved in an aqueous solution, and the pH of the composition is between 1 and 4.
WO 2002/028550 discloses a method of imparting corrosion resistance to a surface of an aluminum substrate, the method comprising bringing the surface of the aluminum substrate into contact with a treating solution containing water and the following components (A) and (B):
(A) 0.1 to 20 mM/kg of a fluorinated compound selected from the group consisting of tetrafluoroboric acid, a partially or completely neutralized water-soluble tetrafluoroborate, hexafluorosilicic acid, a partially or completely neutralized water-soluble hexafluorosilicate, hexafluorotitanic acid, a partially or completely neutralized water-soluble hexafluorotitanate, hexafluorozirconium acid, a partially or completely neutralized water-soluble hexafluorozirconate, hexafluorohafnium acid, a partially or completely nelized water-soluble hexafluorohafnate, and a mixture thereof (the concentration of the salt in the solution being stoichiometrically equivalent to the concentration of the corresponding acid); and
(B) 0.40 to 95 mM/kg of vanadate anions stoichiometrically equivalent to vanadium atoms.
WO 2003/078682 discloses a method for applying a coating to a metallic surface, comprising, in sequence, treating a metallic surface with an aqueous surface-treating agent containing (a) a tungstate ion source and (b) a soluble zirconium-containing material; and drying and/or baking the treated metallic surface.
Metallic components prepared by these treatments exhibit comparatively low electrical resistance in some cases but do not exhibit sufficient corrosion resistance. In addition, the metallic materials applicable are limited to either magnesium components or aluminum components in Japanese Unexamined Patent Application Publication Nos. 2000-96255 and 2002-12980, WO 2003/069024, and WO 2002/028550. Furthermore, it is found that when the aqueous surface-treating agent of WO 2003/078682 is applied to magnesium components, the originally intended purposes, i.e. low electrical resistance and high corrosion resistance are not accomplished.